1. Field of the Invention
This invention relates to motor driven pumps of the canned or isolated stator type.
2. Description of the Prior Art
Motor driven pumps having motor stators in which the stator electrical windings are isolated from the pumped fluids are called "canned" motor pumps and are well known in the art. Such pumps are utilized where corrosive, toxic and expensive fluids are handled and eliminate rotating seals and stuffing boxes which connect the hydraulic apparatus to a motor, which could leak fluids to the atmosphere and cause hazardous conditions and expensive loss of fluids.
An electric motor requires cooling to properly perform, and most standard motors have built-in fans to provide cooling. A canned motor stator that is hermetically sealed cannot be cooled in this fashion and, therefore, some of the fluid that is being handled is recirculated through the motor between the rotor and canned stator for this purpose. This design is common to the U.S. Patents to White U.S. Pat. No. 2,906,208; and to Litzenberg, U.S. Pat. Nos. 2,871,791 and 4,065,237. These patents utilize internal fluid circulation for cooling purposes, but are subject to various limitations and problems.
Many additional designs have been developed which use external fluid circulation, such as the U.S. Patents to White U.S. Pat. Nos. 2,713,311; 3,053,189; and 3,114,090, which are illustrative of that art.
Canned motor pumps that utilize internal circulation depend upon the clearance between the rotor and stator as an orifice for controlling the amount of fluid flow through the motor section. The following difficulties arise from this method of fluid control:
1. The variances in the manufacturing tolerances which constitute this fluid gap are of such a magnitude that either more or less of the desired flow is obtained;
2. The rotor and stator of a canned motor pump vary in length depending upon the horsepower of the pump and this results in an unstable and unpredictable volume of fluid; and
3. The fluid pressure developed in the volute section is completely variable due to the operating conditions and coupled with 1 and 2 above does not provide an accurate flow for the purposes of cooling the motor.
Fluid flow for the purpose of stator cooling must vary with the horsepower developed by the motor, too much causes a drop in efficiency of the machine and too little will reduce the service life of the pump. For example, intensive tests have established that 3 GPM of room temperature fluids is required to properly cool the stator section in a 3 HP motor. Any more fluid is unnecessary and seriously affects the hydraulic efficiency of the unit and any less may be detrimental to the service life of the motor windings. Tests on hundreds of production units have positively demonstrated, that due to the conditions stated above, the use of the clearance between the rotor and stator sections for fluid flow do not control the fluid flow in a satisfactory manner.
The utilization of an orifice of predetermined size will provide the proper fluid flow control since this orifice should be size for the individual horsepower and motor cooling requirements of any size pumps. This orifice eliminates all of the deviations that arise from the use of the gap between the rotor and stator as a metering medium for cooling or other purposes.
To make this orifice effective it is mounted into a plate that has no other openings of consequence from the pump chamber into the stator and rotor section. The orifice is a control orifice to permit the fluid flow to continue to the hollow shaft which returns recirculated fluid to suction or the impeller.
An additional problem to the cooling problem is thrust balancing.
Since canned motor driven pumps of the type illustrated use the clearance between the rotor and stator as a variable orifice for controlling fluid flow, they are very sensitive to electrical forces which tend to center the rotor, and make thrust balancing difficult, since the volume of fluid going through the orifice between the rotor and isolated stator and through the bearings can seriously affect the thrust balance.
The pressure of the pumped fluid is used to operate and control the axial thrust balance of the rotating parts, both electrical and hydraulic. In a single stage closed impeller of a centrifugal pump, a forward thrust is developed due to the differential pressures developed in the front and rear areas of the impeller. The rear plate of the impeller has the fully developed pressures of generated forces across its entire diameter. The front of the impeller has a large suction action of 0 or negative pressure, and a lower pressure from the outside diameter of the impeller to the suction inlet. This low pressure is caused by a front wearing ring which permits escapement of fluids from the impeller front face into the suction.
The differential of the forces on the rear and front of the impeller are of great magnitude which must be neutralized to prevent forward thrust in all other canned motor pumps with the exception of Litzenberg U.S. Pat. No. 4,065,231. In the prior art, thrust washers are provided to handle the thrust forces, however, thrust washers do not neutralize the pressure differential and merely transfer the load to a thrust bearing against which the washer bears. The thrust washer method of handling thrust is inefficient since the washer's action is similar to that of a brake shoe which wastes energy and establishes wearing parts which are costly to make and replace.
Litzenberg and many of the other available designs also utilize fixed non-rotating bearings, which can wear in an egg shaped pattern, resulting in a short service life and requiring frequent replacement. Such bearings are often difficult to replace and may suffer from other problems.
The motor driven pumps of my invention are not subject to the prior art problems, and provide controlled cooling and automatic thrust balancing.